SMTP

NewSMTPSender opens a connection to your SMTP server with TLS or STARTTLS, authenticates, and implements the Sender interface. All knobs — retries, timeouts, rate limits, pooling, DKIM — live in SMTPConfig.

SMTPConfig

Only Host, Port, and credentials are required. Defaults are sensible for most providers; tune them for your throughput and reliability needs.

type SMTPConfig struct {
    // Required
    Host     string // SMTP server hostname
    Port     int    // 587 for STARTTLS, 465 for implicit TLS, 25 for plain
    Username string
    Password string
    From     string // Default sender for Mailer.Send

    // TLS
    UseTLS bool // STARTTLS upgrade after EHLO

    // Reliability
    Timeout      time.Duration // 30s
    MaxRetries   int           // 3
    RetryDelay   time.Duration // 1s
    RetryBackoff float64       // 2.0
    RateLimit    int           // emails/sec, default 10

    // Pooling (PoolSize > 0 enables it)
    PoolSize        int
    MaxIdleConns    int           // 2
    PoolMaxLifetime time.Duration // 30m
    PoolMaxIdleTime time.Duration // 5m
    MaxMessages     int           // per connection, default 100
    PoolWaitTimeout time.Duration // 5s

    // Optional integrations
    Logger Logger
    DKIM   *DKIMConfig
}

Common Providers

SMTP settings for the most common transactional providers:

Gmail

Use an App Password with 2-Step Verification enabled.

email.SMTPConfig{
    Host:     "smtp.gmail.com",
    Port:     587,
    Username: "you@gmail.com",
    Password: "your-app-password",
    UseTLS:   true,
}

SendGrid

email.SMTPConfig{
    Host:     "smtp.sendgrid.net",
    Port:     587,
    Username: "apikey",
    Password: os.Getenv("SENDGRID_API_KEY"),
    UseTLS:   true,
}

AWS SES

email.SMTPConfig{
    Host:     "email-smtp.us-east-1.amazonaws.com",
    Port:     587,
    Username: os.Getenv("SES_SMTP_USERNAME"),
    Password: os.Getenv("SES_SMTP_PASSWORD"),
    UseTLS:   true,
}

Mailgun

email.SMTPConfig{
    Host:     "smtp.mailgun.org",
    Port:     587,
    Username: "postmaster@mg.example.com",
    Password: os.Getenv("MAILGUN_SMTP_PASSWORD"),
    UseTLS:   true,
}

Connection Pooling

For sustained throughput, set PoolSize > 0 to reuse SMTP connections. Without pooling, every send pays the TCP + TLS + AUTH handshake cost (often 200–500ms per message).

config := email.SMTPConfig{
    Host:            "smtp.gmail.com",
    Port:            587,
    Username:        "you@gmail.com",
    Password:        "app-password",
    UseTLS:          true,
    PoolSize:        5,                // up to 5 open connections
    MaxIdleConns:    2,                // keep 2 warm when idle
    PoolMaxLifetime: 30 * time.Minute, // recycle long-lived conns
    MaxMessages:     100,              // rotate after 100 messages
}

sender, err := email.NewSMTPSender(config)
if err != nil {
    log.Fatal(err)
}
defer sender.Close() // closes every pooled connection

Tip: always defer sender.Close() when pooling is enabled — otherwise idle connections leak until the process exits.